this is an easy one! Chipset AMD 780G (wich is the best integrated video chipset out there) with an AMD Athlon X2 4600+
You can build a complete computer, without display and input devices, for around 300$ with 2 gig of dual channel ddr2 Memory, 160GB 7200RPM hard drive. Wich is a verry nice setup for basic computing/internet/media center/office work/etc... even a little gaming.

This would be a pretty sweat basic setup that would run any thing you throw at it flawlessly and be upgradable for a while, since it has PCI-e a PCI-e slot available and an AM2+ socket and available memory slots for 2 more sticks.